In the transmission of digital data on a telephone channel, for example, signal degradation generally occurs on account of thermal noise and interference between adjoining data pulses which may partly overlap, especially if transmission speeds are high. To minimize such distortion, use is conventionally made of filtering networks known as equalizers which are designed to flatten the amplitude characteristic and to linearize the phase characteristic of the transmission channel. Such conventional equalizers, consisting of cascaded passive phase shifters, are based upon the structural characteristics of the signal path and cannot take into account certain factors arising only in operation.
More recent developments, therefore, include the design of adjustable equalizers of the so-called "transverse-filter" type with a response characteristic adaptable to existing operating conditions. These equalizers are put through two successive phases, i.e. a preliminary or acquisition phase and a subsequent operational or working phase. In the acquisition phase the equalizer rapidly adjusts itself, on the basis of a series of predetermined test codes transmitted over the channel and identical reference codes generated locally at the receiving end, while during the working phase it follows the gradual changes in transmission characteristics in response to an error signal fed back from a signal-regenerating unit in its output. Such a signal regenerator, which may be referred to as a decision network, quantizes the data pulses issuing from the equalizer according to predetermined levels of pulse magnitude; the detected differences between the quantized levels and the actual output signal serve for the automatic adjustment of the parameters of the equalizer in a sense tending to reduce the error signal to zero.
Such dynamic equalizers operate generally satisfactorily in the case of relatively low pulse cadences, with little or no superposition of adjoining data pulses; they are not very effective against noise distortion which, e.g. in communication systems utilizing tropospheric or ionospheric channels, can assume greater significance than intercharacter interference. Moreover, their initial adjustment at the start of communication or following temporary loss of signal is relatively slow, calling for a lengthy acquisition phase.